Enclosure

Folklore

Camden, in his "Britannia," informs us, -- "In a rock in the island of Barry, in Glamorganshire, there is a narrow chink, or cleft, to which if you put your ear you shall perceive all such sorts of noises as you may fancy smiths at work under ground, strokes of hammers, blowing of bellows, grinding of tools." At Worm's head, in the peninsular of Gower in Glamorganshire, these sounds are, even now, often heard; and it requires but a moderate stretch of imagination to create all this cyclopean imagery, when the sea is rolling in cavities under our feet, and the tone of its voice is magnified by confinement and repercussion.

From p151 of 'The Philosophy of Mystery' by Walter Cooper Dendy (1841).

I've since found out it wasn't Camden at all - it was Giraldus Cambrensis:

.. In a rock or cliff, by the sea=side in Glamorganshire, near the Isle of Barry, there appeareth a little chink, into which, if you lay your ear, you shall hear a noise as of smiths at work - one while the blowing of bellows, another while striking of sledge and hammer, sometime the sound of the grindstone and iron tools rubbing against it, the hissing sparks also of steel gads within holes as they are beaten, yea and the puffing noise of fire burning in the furnace. Now I am persuaded that the sound comes of the rush of the sea water.